Passive needs changing. At the moment, the range is only useful for encounters with melees, but as soon as someone comes within melee he becomes insane strong. My suggestion:
By the Shield(Innate): Cain gains 4/5/6 armor and magic resistance for each allied champion within 800 range and grants half of this amount to such allies.
500 Range is too small. Only helps when really close to a teammate, REALLY, so early game strength was extremely weak, very situational.
To counteract the range enhancement, which turned the passive from almost never to standard, the values needed to be lowered a tad.
Granting the bonus to all allies seemed to strong, so I decided that it should probably only be to the allies effecting it.
For those that think the armour levels are too low late game, think about it this way. In a team fight you would probably have about 2 allies in the range. The old passive would grant 24 for you, and 12 for the rest. Now, it would be easier to have all 4 allies in range. 24 for you, 12 for the rest.
The maximum for the old one was 48 bonus armour, which I thought was too much. If Cain built tank it would barely matter for him, but his squishy allies would get 24 armour, which is quite massive.